Texture atlas

* Combines `TextureAtlas` and `TextureAtlasBuilder`. 
* Makes `TextureAtlas` and `BillboardCollection.textureAtlas` private.
* Removes `Billboard.imageIndex` and adds `Billboard.imageUrl` and `Billboard.image`.

For #1688.

The functions in `TextureAtlas` were changed to mimic those that were in `TextureAtlasBuilder` though it may be better to return promises. See the use of the `TextureAtlas` in `LabelCollection` which expects the callbacks to be executed before returning.


Why have the callback here?  Why not take the canvas and friends similar to how [Texture](https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/Renderer/Texture.js#L113) works?

I almost would expect the callback to be called every frame as if this was for dynamic billboards.